1. Field of the Invention
The present invention relates to an optical digital logical operation circuit for use in optical communication systems, optical data processing systems, optical switching systems and optical computers, and more particularly to an optical delay (D) flipflop and a logical operation circuit using it.
2. Prior Art
With the progress of optical fiber and optical semiconductor technologies in recent years, long-haul optical communication systems have become practical and optical LAN systems for highly efficient connection of distributed processing systems.
In these systems, optical techniques are mainly used for the interconnection of functional units. Such units employ electronic circuitry which is fabricated by large-scale integrated circuits (LSIs).
With the further diversification and increasing volume of information to be handled by such systems, reflecting the development of a highly information-oriented society in the near future, requirements are growing for even faster and more complex processing of information.
To meet these requirements, it has become necessary to utilize optical techniques not only as connecting means but also as logical processing means.
Accordingly optical processing systems (including optical data processing systems and optical switching systems), which fully utilize the high speed, broad bandwidth and induction-immunity of light, are required. In such systems, it is necessary to digitally process optical digital signals as they are, and to optically transmit the results of such processing to other systems. To realize such systems, optical logic functional blocks like those used in common electric logical operation circuits must be designed.
In particular, optical flipflops and optical shift registers, whose components include such flipflops, are important optical logic blocks that will be used in composing an optical processing system.
In the prior art, optical logical operation circuits are hybrids of optical circuits and electronic circuits. Thus a typical such device receives optical digital data with a light receiving element, converts the optical data to electrical data for processing with an electronic circuit, and then drives a light emitting element to provide output optical digital data.
For example in an article, "A Microprocessor and Optoelectronics Based Packet Switch for Satellite Communications" in the Conference Record of the IEEE International Conference on Communications, pp. 15.3.1 to 15.3.5, 1981, is disclosed an optical shift register. In this shift register, each bit is stored in a combination of a photodiode, a light emitting diode (LED), connected in series, and a 1.times.2 optical switch. Optical digital data are received by the photodiode and are converted into electric signals, which actuate the LED. The light output from the LED, when the 1.times.2 optical switch is in crossover mode, excites the photodiode of the current bit, and then excites the LED of the current bit to achieve a self-sustained state. On the other hand, when the 1.times.2 optical switch is in parallel mode, the light output from the LED excites the photodiode of the next bit. Said optical switch is clock-controlled, and the content of each bit is transferred to the immediately following bit.
The performance characteristics of these prior art structures, wherein optical digital data are processed through electronic circuits, are affected by the performance characteristics of the electric circuits. Thus the stray inductance and the stray capacitance resulting from the composition of such an electronic circuit will deteriorate the bandwidth of the device thereby adversely affecting the high speed and broad bandwidth of optical signals. Accordingly it is difficult to realize a light processing system, to which a broad bandwidth and high speed are important, by the use of conventional optical flipflops and optical shift registers.